[Server] Enhance role permission validation

[romanett]

• Make ValidateRolePermissions virtual in CustomNodeManager.cs to allow custom validation
• Initialize OperationContext in Node.ReadAttribute of MonitoredNode to allow custom validation
• Update ValidateRolePermissions in MasterNodeManager.cs to prevent null reference exception

Related Issues

• Fixes NullReferenceException in Opc.Ua.Server.MasterNodeManager.ValidateRolePermissions() #3046

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which adds functionality)
• Test enhancement (non-breaking change to increase test coverage)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, requires version increase of Nuget packages)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc.
• I have signed the CLA.
• I ran tests locally with my changes, all passed.
• I fixed all failing tests in the CI pipelines.
• I fixed all introduced issues with CodeQL and LGTM.
• I have added tests that prove my fix is effective or that my feature works and increased code coverage.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 57.05%. Comparing base (2f0770c) to head (99d3ba5).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
...ies/Opc.Ua.Server/NodeManager/MasterNodeManager.cs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3047      +/-   ##
==========================================
+ Coverage   56.81%   57.05%   +0.23%     
==========================================
  Files         357      357              
  Lines       68862    68978     +116     
  Branches    14133    14160      +27     
==========================================
+ Hits        39123    39354     +231     
+ Misses      25541    25377     -164     
- Partials     4198     4247      +49     

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[mrsuciu]

Looks good except the "potential" resource consumption

[mrsuciu]

See comments

[mrsuciu]

Looks good